Let me try to explain. First, since "eval" is valid Perl, and so are the & and && operators, Perl will never execute any of it, because $running_under_some_shell is false.

$?0 is special csh construct, returning 1 if the current filename is known, 0 otherwise. $? is the exit status of the most recent pipeline in sh. Since (exit $?0) is executed in a subshell (due to the parens), and from a file, eval '(exit $?0)' will return 0 in sh, but 1 in csh. This means, the eval 'exec perl -wS $0 ${1+"$@"}' is only executed by sh. But when executed by sh, sh will not see anything else of the file.

Which leaves use with & eval 'exec /usr/bin/perl -wS $0 $argv:q'. I do not quite know how csh parses that - I can only assume csh allows a "null command" - which is then put in the background. Then it executes /usr/bin/perl -wS $0 $argv:q. We already know the meaning of the -wS $0 part. $argv is a variable holding all the arguments to the program. The :q is a modifier, quoting all the arguments (just like "$@" does in sh).